Method of recycling a delaminated wafer and a silicon wafer used for the recycling

ABSTRACT

There is disclosed a method of recycling a delaminated wafer produced as a by-product in producing an SOI wafer according to a hydrogen ion delaminating method by reprocessing it for reuse as a silicon wafer, wherein at least polishing of the delaminated wafer for removing of a step in the peripheral part of the delaminated wafer and heat treatment in a reducing atmosphere containing hydrogen are conducted as the reprocessing. There are provided a method of appropriately reprocessing a delaminated wafer produced as a by-product in a hydrogen ion delaminating method to reuse it as a silicon wafer actually, and particularly, a method of reprocessing an expensive wafer such as an epitaxial wafer many times for reuse, to improve productivity of SOI wafer having a high quality SOI layer, and to reduce producing cost.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of recycling a siliconwafer by reprocessing a delaminated (split) wafer produced as aby-product in a hydrogen ion delaminating method (also called a smartcut method) wherein an ion-implanted wafer is bonded to another waferand a portion of the ion-implanted wafer is delaminated to therebyobtain an SOI (silicon on insulator) wafer. Particularly, the presentinvention relates to a method of reprocessing the delaminated wafer manytimes, and reusing it many times.

[0003] 2. Description of the Related Art

[0004] Conventionally, two methods have gained wide notice as methods offabricating wafers having an SOI structure. One method is a SIMOX(separation by implanted oxygen) method in which oxygen ions areimplanted into a silicon monocrystal at a high concentration, and heattreatment is then performed at a high temperature in order to form anoxide layer. The other method is a bonding method in which twomirror-polished silicon wafers are bonded together without use ofadhesive, and one of the wafers is subsequently made very thin.

[0005] In the SIMOX method, the thickness of an SOI layer that becomes adevice active region can be determined and controlled through adjustmentof an acceleration voltage at the time of oxygen ion implantation.Therefore, the SIMOX method has an advantage of enabling easy formationof a thin SOI layer having a high uniformity of thickness. However, theSIMOX method has many problems in relation to the reliability of aburied oxide layer, the crystallinity of the SOI layer, necessity ofheat treatment at a temperature of 1300° C. or higher, and the like.

[0006] Meanwhile, in the wafer bonding method, an oxide film is formedon at least one of two mirror-polished silicon single crystal wafers,which are bonded together without use of adhesive and then subjected toheat treatment (typically, at 1100-1200° C.) in order to strengthen thebonding; subsequently, one of the wafers is subjected to grinding or wetetching such that the wafer becomes a thin film, the surface of which isthen mirror-polished to form an SOI layer. Therefore, the reliability ofthe buried oxide layer is high, and the crystallinity of the SOI layeris good.

[0007] However, since the thin film is formed by means of mechanicalmachining, it takes long time to form the thin film, and a great portionof one of the wafers is pulverized to be lost. Therefore, productivityis extremely low, and production cost is significantly high.Furthermore, there are limits to the thickness and thickness uniformityof the resultant SOI layer in formation of the thin film by grinding andpolishing by means of mechanical machining.

[0008] Furthermore, most of waters used in the wafer bonding method areCZ wafers produced by Czochralski method (CZ method). However, it hasbeen found in recent years that there exist in the CZ wafer crystaldefects called COP (Crystal Originated Particle) which are incorporatedwhile the crystal is growing. Accordingly, when the CZ wafer is used asa bond wafer which is to be a device active layer, COP exists also inthe SOI layer, and pierces through the SOI layer which is very thin asrequired recently to form a pin hole which extremely degrades electriccharacteristics of the wafer.

[0009] To solve the problem, there is proposed, for example, a methodwherein CZ wafer on which an epitaxial layer is grown is bonded to otherwafer on the side of the epitaxial layer, and the silicon wafer whichconstitutes a base is ground and polished to form a SOI layer (seeJapanese Patent Application Laid-open (Kokai) No. 7-254689). Accordingto the method, the above mentioned crystal defects such as COP can besurely eliminated. However, since it is necessary to use an expensiveepitaxial wafer, production cost of the SOI wafer gets far higher.

[0010] When FZ wafer is used, there is no problem in relation to defectsdue to oxygen or the above mentioned COP, since no oxygen is containedin the FZ wafer. However, since only one SOI wafer can be obtained fromtwo silicon wafers, there still exists the problem of high productioncost.

[0011] In the wafer bonding method, not only silicon wafers are bondedtogether, but also a silicon wafer may be bonded directly to aninsulator wafer of SiO₂, SiC, Al₂O₃ or the like, in order to form an SOIlayer.

[0012] Recently, public attention has been drawn to a new method offabricating an SOI wafer in which an ion-implanted wafer is bonded toanother wafer and a portion of the ion-implanted wafer is delaminated(split) to thereby obtain an SOI wafer (hydrogen ion delaminatingmethod: so-called smart-cut method). In this method, an oxide film isformed on the surface of at least one of two silicon wafers; andhydrogen ions or rare gas ions are implanted into the surface of one ofthe two silicon wafers in order to form a fine bubble layer (enclosedlayer) within the wafer; the ion-implanted silicon wafer is superposedon the other silicon wafer such that the ion-implanted surface comesinto close contact with the surface of the other silicon wafer via theoxide film; heat treatment is performed to delaminate a portion of theion-implanted wafer while the fine bubble layer is used as adelaminating plane, in order to form a thin film; and heat treatment(bonding heat treatment) is further performed to firmly bond the thinfilm and the other wafer, to thereby obtain an SOI wafer (see JapanesePatent Application Laid-Open (kokai) No. 5-211128 or U.S. Pat. No.5,374,564). Also, in this method, since the surface formed as a resultof delamination is a good mirror-like surface, an SOI wafer whose SOIlayer has a high thickness uniformity is obtained with relative ease.

[0013] Also in the hydrogen ion delaminating method described above, notonly silicon wafers are bonded together, but also an ion-implantedsilicon wafer may be bonded directly to an insulator wafer of SiO₂, SiC,Al₂O₃, etc., in order to form an SOI layer.

[0014] When the SOI wafer is fabricated by the hydrogen ion delaminatingmethod, a silicon delaminated wafer is inevitably produced as aby-product. It has been proposed to reuse such a delaminated wafer as aby-product so that one SOI wafer can be obtained substantially from onesilicon wafer, and thereby to reduce production cost significantly.

[0015] It is conceptually possible to reuse the delaminated wafer, butthere has been no actual cases of reusing the delaminated wafer, and thespecific means for reusing it has not yet been known. The inventors ofthe present invention have studied and found that the wafer as justdelaminated cannot be reused for the following reasons: a step ispresent in the peripheral part of the wafer; a damage layer due to ionimplantation is present on the surface thereof; and surface roughnessthereof is large. Accordingly, it is necessary to remove the step in theperipheral part of the wafer, the damage layer, or the like, in order toreuse the delaminated wafer as a silicon wafer.

[0016] In this case, it may be conceived that the surface of thedelaminated wafer is ground, and then polished in order to remove thestep in the peripheral part and the damage layer, and to improve surfaceroughness or the like. However, in order to improve the surfaceproperties by grinding and polishing, long processing time and a lot ofstock removal are required. Moreover, in the case that an epitaxiallayer is used as a bond wafer in the hydrogen ion delimitation method,and SOI layer is made of an epitaxial layer, since the epitaxial layerremaining on the delaminated layer is completely removed, the epitaxiallayer cannot be reused as a SOI layer. Accordingly, cost for fabricationof SOI wafer cannot be reduced.

[0017] In the case that CZ wafer is used as a bond wafer, the problemsin relation to the above-mentioned COP cannot be solved by grinding andpolishing. In the case that FZ wafer is used as a bond wafer, a lot ofstock removal in grinding or polishing means the small number of timesof reuse, and further, long processing time, and high production cost.

SUMMARY OF THE INVENTION

[0018] The present invention has been accomplished to solve theabove-mentioned problems, and an object of the present invention is toprovide a method of appropriately reprocessing a delaminated waferproduced as a by-product in a hydrogen ion delaminating method to reuseit as a silicon wafer actually. Particularly, the object of the presentinvention is to provide a method of reprocessing an expensive wafer suchas an epitaxial wafer many times for reuse, to improve productivity ofSOI wafer having a high quality SOI layer, and to reduce producing cost.

[0019] To achieve the above object, the present invention provides amethod of recycling a delaminated wafer produced as a by-product inproducing an SOI wafer according to a hydrogen ion delaminating methodby reprocessing it for reuse as a silicon wafer wherein polishing of thedelaminated wafer for removing of a step in the peripheral part of thedelaminated wafer and heat treatment in a reducing atmosphere containinghydrogen are at least conducted as the reprocessing.

[0020] As described above, it has been found that there is a step in theperipheral part of the delaminated wafer produced as a by-product.According to the method of the present invention, the step in theperipheral part of the delaminated wafer is removed by polishing asreprocessing. With polishing for removal of the step in the peripheralpart of the delaminated wafer, the peripheral step can be easilyremoved, and moreover, the damage layer on the surface of thedelaminated wafer can also be removed, and the surface roughness canalso be improved at the same time.

[0021] Basically, polishing is conducted only to the stock removalnecessary for removal of the step in the peripheral part in the presentinvention. Removal of the damage layer remaining on the surface andimprovement of the surface roughness are achieved by the heat treatmentfor the delaminated wafer in a reducing atmosphere containing hydrogen.

[0022] The present invention also provides a method of recycling adelaminated wafer produced as a by-product in producing an SOI waferaccording to a hydrogen ion delaminating method in which an epitaxialwafer is used as a bond wafer by reprocessing it for reuse as a siliconwafer wherein polishing of the delaminated wafer for removal of a stepin the peripheral part of the delaminated wafer and heat treatment in areducing atmosphere containing hydrogen are at least conducted as thereprocessing.

[0023] As described above, since the stock removal is kept to a minimumto reprocess the delaminated wafer in the method of the presentinvention, the epitaxial layer of the expensive epitaxial wafer can bereprocessed and reused many times, so that the SOI wafer of high qualitycan be fabricated at low cost.

[0024] Furthermore, the present invention also provides a method ofrecycling a delaminated wafer produced as a by-product in producing anSOI wafer according to a hydrogen ion delaminating method in which CZwafer is used as a bond wafer by reprocessing it for reuse as a siliconwafer wherein polishing of the delaminated wafer for removing of a stepin the peripheral part of the delaminated wafer and heat treatment in areducing atmosphere containing hydrogen are at least conducted as thereprocessing.

[0025] As described above, since the delaminated wafer is subjected tothe heat treatment in a reducing atmosphere containing hydrogen in themethod of the present invention, a damage layer on the surface of thedelaminated wafer can be removed, the surface roughness can be improved,and further, COPs in CZ wafer can be eliminated. Accordingly, in thecase that CZ wafer is used as a bond wafer in a hydrogen iondelamination method, and a delaminated wafer is CZ wafer, the recycled(reclaimed) wafer in which COPs in the delaminated wafer are eliminatedcan be obtained.

[0026] Moreover, the present invention provides a method of recycling adelaminated wafer produced as a by-product in producing an SOI waferaccording to a hydrogen ion delaminating method in which FZ wafer isused as a bond wafer by reprocessing it for reuse as a silicon waferwherein polishing of the delaminated wafer for removing of a step in theperipheral part of the delaminated wafer and heat treatment in areducing atmosphere containing hydrogen are at least conducted as thereprocessing.

[0027] As described above, since the stock removal is kept to a minimumto reprocess the delaminated wafer in the method of the presentinvention, the FZ wafer having no COP can be reprocessed many times tobe reused, so that the SOI wafer having high quality can be fabricatedat low cost.

[0028] It is preferable that a surface oxide film is removed beforepolishing for removal of a step in the peripheral part, as reprocessingof delaminated wafer.

[0029] When the surface oxide film is removed before polishing forremoval of the peripheral step, the polishing can be conducteduniformly. Namely, when the oxide film is present on the peripheral parthaving the step, the step will be larger. Furthermore, since the oxidefilm and the silicon are different in hardness, uniform polishing isdifficult in the presence of the oxide film.

[0030] A stock removal in polishing for removal of the step in theperipheral part as reprocessing of the delaminated wafer is preferably 1to 2 μm.

[0031] With such a small stock removal, the peripheral step can besurely removed. Furthermore, since the stock removal is small, theepitaxial layer can be used as the SOI layer many times by reprocessingthe delaminated wafer, even when the epitaxial wafer is used as a bondwafer.

[0032] It is preferable that the heat treatment in a reducing atmospherecontaining hydrogen as reprocessing of the delaminated wafer isperformed at a temperature in the range from 1000° C. to a melting pointof silicon, for 6 hours or less.

[0033] When the heat treatment is performed under such a condition,namely at high temperature for a long time, the damage layer on thesurface can be removed and surface roughness can be improved surely.Furthermore, the heat treatment in a reducing atmosphere containinghydrogen can be performed through use of a conventional resistanceheating furnace.

[0034] In another embodiment of the present invention, the heattreatment as reprocessing of the delaminated wafer in a reducingatmosphere containing hydrogen is performed at a temperature in therange from 1000° C. to a melting point of silicon for 1 to 300 secondsthrough use of a rapid heating/rapid cooling apparatus.

[0035] When the delaminated wafer is subjected to the heat treatment ina reducing atmosphere containing hydrogen through use of the rapidheating/rapid cooling apparatus as mentioned above, removal of a damagelayer on the surface of the delaminated wafer and improvement of surfaceroughness thereof can be achieved efficiently in a very short time.

[0036] In another embodiment of the present invention, the heattreatment as reprocessing of the delaminated wafer in a reducingatmosphere containing hydrogen is performed in 100% hydrogen atmosphereor a mixed atmosphere of hydrogen and argon.

[0037] In such an atmosphere for the heat treatment, the surface of thedelaminated wafer can be surely improved.

[0038] As described above, the delaminated wafer reprocessed accordingto the present invention has high quality, namely, the peripheral stepcan be removed, the damage layer can be removed and the surfaceroughness can be improved therein. Therefore, it can be used as varioussilicon wafers.

[0039] Particularly, according to the present invention, since the stockremoval can be decreased, the reprocessed wafer can be reused as a bondwafer of the SOI wafer, and thereby the delaminated wafer can be reusedfor fabrication of the SOI wafer many times.

[0040] The present invention also provides a silicon wafer to be reused,which is obtained according to the above-mentioned methods ofreprocessing.

[0041] As described above, the delaminated wafer reprocessed accordingto the present invention can be reused as a silicon wafer. Particularly,since the stock removal for reprocessing according to the presentinvention is very small, it is not always necessary to previouslycontrol thickness of the wafer to be delaminated in the hydrogen iondelaminating method. Accordingly, recycling of the delaminated wafer canbe conducted quite easily and conveniently.

[0042] As described above, when the delaminated wafer produced as aby-product in the hydrogen ion delaminating method is reprocessedappropriately according to the present invention, it can be actuallyreused as a silicon wafer. Particularly, the stock removal of thedelaminated wafer can be decreased, an expensive wafer such as anepitaxial wafer can be reprocessed many times to be reused many times.Thereby, improvement in productivity of the SOI wafer having the SOIlayer of high quality and cost reduction can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 (a)-(h) is a flowchart showing an example of an SOI-waferfabricating process according to a hydrogen ion delaminating method.

[0044]FIG. 2 (A)-(F) is a flowchart showing a method of reprocessing adelaminated wafer for reuse of the present invention, employed in anexample.

[0045]FIG. 3 is a graph showing a result of measurement of a peripheralstep of a delaminated wafer.

[0046] (A) shows a delaminated wafer which is not processed,

[0047] (B) shows a delaminated wafer after an oxide film is removed, and

[0048] (C) shows a delaminated wafer after a peripheral step is removed.

[0049]FIG. 4 is a graph showing a relation between stock removal inpolishing for removal of a peripheral step and height of the step.

[0050]FIG. 5 is a graph showing a result of measurement of a damagelayer of the delaminated wafer.

[0051]FIG. 6 is a schematic view of an example of a rapid heating/rapidcooling apparatus.

DESCRIPTION OF THE INVENTION AND A PREFERRED EMBODIMENT

[0052] The present invention will be further described below in detail,referring to the drawings, but is not limited thereto.

[0053] The present invention will be hereunder explained mainly as for amethod of producing SOI wafer by bonding two silicon wafers.

[0054] In step (a) of the hydrogen ion delaminating method shown in FIG.1, two silicon wafers, namely a base wafer 1 to be a base and a bondwafer 2 to be an SOI layer which are suitable for device specificationsare prepared. In this embodiment, an epitaxial wafer consisting of asilicon mirror wafer on which an epitaxial layer 13 having a thicknessof about 10 μm was grown is employed as a bond wafer 2.

[0055] In step (b), at least one of the wafers, the bond wafer(epitaxial wafer) 2 in this case is subjected to thermal oxidation so asto form on the surface thereof an oxide film 3 having a thickness ofabout 0.1 μm to 2.0 μm.

[0056] In step (c), hydrogen ions or rare gas ions are implanted intoone surface, on which the epitaxial layer is formed, of the bond wafer 2on which oxide film is formed, in order to form a fine bubble layer(enclosed layer) 4 which extends in parallel to the surface at aposition corresponding to the mean penetration depth of ionimplantation. The implantation temperature is preferably 25-450° C.

[0057] In step (d), the base wafer 1 is superposed on the hydrogenion-implanted surface (the epitaxial layer formed surface) of thehydrogen ion-implanted bond wafer 2 via the oxide film, and they arebrought in close contact with each other. When the surfaces of the twowafers are brought into contact with each other at ambient temperaturein a clean atmosphere, the wafers adhere to each other without use ofadhesive or the like.

[0058] In step (e), there is performed a heat treatment for delaminationin which a delaminated wafer 5 is delaminated from an SOI wafer 6(composed of the SOI layer 7, a buried oxide layer 3, and a base wafer1) while the enclosed layer 4 is used as a delaminating plane. The heattreatment is performed, for example, at a temperature of about 500° C.or higher in an inert gas atmosphere so as to cause crystalrearrangement and bubble cohesion, and thereby the delaminated wafer 5is delaminated from the SOI wafer 6. In this case all of the SOI layer 7is made of an epitaxial layer.

[0059] In step (f), the SOI wafer 6 is subjected to heat treatment athigh temperature as bonding heat treatment to achieve sufficient bondingstrength, since the bonding strength imparted in the closely contactingstep (d) and the delaminating heat treatment step (e) described above isnot sufficient to be used without further treatment in a device process.Preferably, this heat treatment is performed, for example, in an inertgas atmosphere at 1050 -1200° C. for 30 minutes to 2 hours.

[0060] Step (e), i.e. the delaminating heat treatment and step (f), i.e.the bonding heat treatment can be conducted successively. Alternatively,step (e), i.e. the delaminating heat treatment he can also serve as abonding heat treatment of step (f).

[0061] In step (g), a mirror polishing process called touch polishingwherein a stock removal is very small is then performed to remove acrystal defect layer on a delaminated surface which is a surface of theSOI layer 7 and improve the surface roughness.

[0062] The SOI wafer 6 of high quality having the SOI layer 7 consistingof an epitaxial layer of high crystal quality and high thicknessuniformity can be produced (step (h)) through the steps described above.

[0063] In the above hydrogen ion delaminating method, the delaminatedwafer 5 is produced as a by-product in the step (e) of FIG. 1. Thethickness of the SOI layer produced by the hydrogen ion delaminatingmethod is generally 0.1 to 1.5 microns, and 2 microns at the thickest.Accordingly, the delaminated wafer 5 has a sufficient thickness.Therefore, production cost for the SOI layer can be significantlyreduced by reusing the delaminated wafer as a silicon wafer.Particularly, when the epitaxial wafer having the epitaxial layer withthickness of about 10 microns is used as a bond wafer as in theabove-mentioned embodiment, the epitaxial layer with thickness of about8 microns or more remains after delamination. Accordingly, if it can beused as a bond wafer again, production cost for the SOI wafer whereinthe epitaxial wafer is used can be significantly reduced.

[0064] However, a step 10 is formed in the peripheral part of thedelaminated wafer 5 as shown in the enlarged schematic view of thedelaminated wafer of FIG. 2(A), and therefore, the wafer cannot be usedas a silicon wafer without processing further. The peripheral step 10 isformed because the periphery of the bond wafer cannot be bonded to abase wafer. Accordingly, the height of the step is approximately thesame as the sum of the thickness of the SOI layer and the thickness ofthe buried oxide layer 3.

[0065] Furthermore, the damage layer 12 due to hydrogen ion implantationremains on the delaminated surface 11 of the delaminated wafer, and thesurface roughness is worse than that of a general mirror wafer.Particularly, the surface roughness is locally inferior. Accordingly,when the wafer is subjected to preferential etching such as alkalietching, deep pits are formed therein.

[0066] In that case, it would be conceived that all of the peripheralstep, the damage layer and the surface roughness is removed by grindingand polishing. However, stock removal will be too much, and processingtime will be too long in such a method. Furthermore, in the case thatthe epitaxial wafer is used as a bond wafer as in the above-mentionedembodiment, the epitaxial layer will be completely removed, andtherefore it will not be able to be reused as an SOI layer.

[0067] In order to solve the above-mentioned problems, the inventorshave studied a method wherein a delaminated wafer produced as aby-product in a hydrogen ion delamination method is reprocessedappropriately to be actually reused as a silicon wafer, and completedthe present invention.

[0068] In particular, the inventors studied a method wherein a stockremoval for reprocessing a delaminated wafer is reduced in order toreprocess and to reuse an expensive wafer with high quality, such as theabove-mentioned epitaxial wafer, many times.

[0069] Namely, according to the present invention, the peripheral stepformed on the delaminated wafer produced as a by-product in fabricationof an SOI wafer by the hydrogen ion delaminating method is removed bybeing polished slightly.

[0070] As described above, when the peripheral step in the delaminatedwafer is removed by polishing, the peripheral step can be easilyremoved. For example, when the thickness of the SOI layer is 0.2 micron,a stock removal of about 1 micron will be enough to remove the stepcompletely. Even when the SOI layer is thicker, a stock removal of 2microns will be enough to remove the peripheral step.

[0071] Furthermore, when the peripheral step is removed by polishing,removal of the damage layer and improvement of the surface roughness onthe delaminated wafer can also be achieved in some extent at the sametime.

[0072] In the method of the present invention, a surface oxide film 3 ispreferably removed in reprocessing of the delaminated wafer before thepolishing for removal of the peripheral step.

[0073] Because, when the surface oxide film 3 is removed beforepolishing for removal of the peripheral step 10, the polishing can beperformed uniformly. When the oxide film 3 is deposited on theperipheral step 10, the height of the step is higher, and uniformpolishing in the surface of the delaminated wafer is difficult becauseof the difference in hardness between the oxide film and silicon.

[0074] The removal of the oxide film can be easily performed by dippingthe delaminated wafer into hydrofluoric acid.

[0075] Next, in the present invention, the delaminated wafer issubjected to the heat treatment in a reducing atmosphere containinghydrogen in order to improve the surface roughness completely. Because,when the finish polishing is performed after polishing for removal ofthe peripheral step, the stock removal will be increased. Therefore,improvement of the surface roughness and removal of the damage layer areachieved by subjecting the delaminated wafer to the heat treatment in areducing atmosphere containing hydrogen as reprocessing for thedelaminated wafer in the present invention.

[0076] The heat treatment can remove the damage layer and improve thesurface roughness without decreasing the thickness of the delaminatedwafer. Accordingly, the stock removal in reprocessing can be basicallykept within the amount which is necessary for removal of the peripheralstep.

[0077] In that case, the order of the polishing for removal of theperipheral step and the heat treatment in a reducing atmospherecontaining hydrogen is not necessary to be specific. It is preferable toremove the peripheral step before the heat treatment, namely to performthe heat treatment after removal of the surface damage layer and thesurface roughness in some extent, since in that case the damage layercan be removed more completely, and the surface roughness can be furtherimproved. However, it doesn't matter if the polishing for removal of theperipheral step is performed after the heat treatment in a reducingatmosphere containing hydrogen.

[0078] The heat treatment in a reducing atmosphere containing hydrogenin the present invention can be conducted preferably, for example, at1000° C. to a melting point of silicon, more preferably at 1200 to 1350°C., for 6 hours or less.

[0079] When the high temperature heat treatment is conducted for a longtime under the above-mentioned condition, it is possible to surelyremove the damage layer on the surface of the delaminated wafer and toimprove the surface roughness with any type of heat treatment furnace.The heat treatment at a high temperature such as 1200° C. or more makesit possible to remove the damage layer efficiently and to improve thesurface roughness. However, heat treatment at a temperature higher than1350° C. may cause problems in relation to durability of a furnace andwafer contamination. Accordingly, the heat treatment at a temperature of1200° C. to 1350° C. is preferable.

[0080] However, the heat treatment using a general heat treatmentfurnace such as described above takes long time. Accordingly, the heattreatment of the present invention can be conducted through use of arapid heating/rapid cooling apparatus at a temperature in the range from1000° C. to the melting point of silicon for 1 to 300 seconds.

[0081] When the delaminated wafer is subjected to the heat treatment ina reducing atmosphere containing hydrogen through use of the rapidheating/rapid cooling apparatus as described above, the damage layer onthe surface of the delaminated wafer can be removed and the surfaceroughness can be improved efficiently in very short time. Temperature inthe range from 1200° C. to 1350° C. is more effective also in this case.

[0082] Examples of an apparatus which can heat and cool the delaminatedwafer rapidly, used in the present invention include: an apparatusheater such as a lamp heater with heat radiating. An example ofcommercially available apparatuses is SHS-2800 (product of AST corp.).These apparatuses are neither extremely complicated nor expensive.

[0083] One example of the apparatus which can heat and cool thedelaminated wafer rapidly, used in the present invention will be givenbelow. FIG. 6 is a schematic sectional view of the rapid heating/rapidcooling apparatus.

[0084] A heat-treatment apparatus 20 shown in FIG. 6 includes a bell jar21 which is formed from, for example, silicon carbide or quarts and inwhich a wafer is heat-treated. Heating is performed with heaters 22 and22′ surrounding the bell jar 21. The heaters are separated into theupper one and the lower one, so that power supplied to each of theheaters can be controlled independently. The heating method is notlimited thereto, but so-called radiation heating and high-frequencyheating may also be applicable. A housing 23 for heat shield is disposedaround the heaters 22 and 22′.

[0085] A water-cooled chamber 24 and a base plate 25 are arranged at thelower portion of a furnace so as to isolate the interior of the bell jar21 from the atmosphere. A delaminated wafer 28 is held on a stage 27,which is attached to the top-end of a support shaft 26 capable of beingmoved vertically by means of a motor 29. In order to load a wafer intoor unload it from the furnace along a horizontal direction, thewater-cooled chamber 24 has an unillustrated wafer port which is openedand closed by means of a gate valve. A gas inlet and a gas outlet areprovided in the base plate 25 so that the gas atmosphere within thefurnace can be adjusted.

[0086] In the heat treatment furnace 20 having the above-describedstructure, the heat treatment for rapid heating/rapid cooling of thedelaminated wafer is carried out in the procedure described below.

[0087] First, the interior of the bell jar 21 is heated to a desiredtemperature of 1000° C. to the melting point of silicon by the heaters22 and 22′ and is then held at the desired temperature. Through mutuallyindependent control on power supplied to the heaters 22 and 22′, atemperature distribution can be established within the bell jar 21 alonga vertical direction. Accordingly, the heat-treatment temperature of awafer is determined by the position of the stage 27, i.e. the amount ofinsertion of the support shaft 26 into the furnace. Heat treatmentatmosphere is controlled by introducing a reducing gas containinghydrogen from the gas inlet provided in the base plate.

[0088] In a state in which the interior of the bell jar 21 is maintainedat a desired temperature, a delaminated wafer is inserted into thewater-cooled chamber 24 through the wafer port by an unillustrated waferhandling apparatus arranged next to the heat treatment apparatus 20. Theinserted wafer is placed in, for example, a SiC boat provided on thestage 27 which is situated at the bottom standby position. Since thewater-cooled chamber 24 and the base plate 25 are water-cooled, thewafer located at this standby position is not heated to a hightemperature.

[0089] Upon completion of placing the delaminated wafer on the stage 27,the motor 29 is immediately driven to insert the support shaft 26 intothe furnace so that the stage 27 is raised to a heat treatment positionwhere a desired temperature in the range of 1000° C. to a melting pointof silicon is established, thereby heat-treating the delaminated waferon the stage at the high temperature. In this case, since onlyapproximately 20 seconds, for example, is required for moving the stage27 from the bottom standby position in the wafer-cooled chamber 24 tothe heat treatment position, the delaminated wafer is heated quickly.

[0090] The stage 27 is halted at the desired temperature position for apredetermined time (1 to 300 seconds), thereby subjecting thedelaminated wafer to high-temperature heat treatment over the haltingtime in a reducing atmosphere. Upon elapse of the predetermined time tocomplete high-temperature heat treatment, the motor 29 is immediatelydriven to-withdraw the support shaft 26 from the interior of the furnaceto thereby lower the stage 27 to the bottom standby position in thewatercooled chamber 24. This lowering motion can be completed inapproximately 20 seconds, for example. The delaminated wafer on thestage 27 is quickly cooled, since the water-cooled chamber 24 and thebase plate 25 are water-cooled. Finally, the delaminated wafer isunloaded from inside the water-cooled chamber 24 by the wafer handlingapparatus, thus completing the heat treatment.

[0091] When there are more delaminated wafers to be heat-treated, thesewafers can be sequentially loaded into and heat-treated in the heattreatment furnace 20 maintained at a predetermined high temperaturewithout lowering temperature.

[0092] In this case, atmosphere of the heat treatment in reducingatmosphere containing hydrogen can be a 100% hydrogen atmosphere or amixed atmosphere of hydrogen and argon.

[0093] In the above-mentioned atmosphere for the heat treatment, a filmwhich is harmful for the surface of the delaminated wafer is not formed,a damage layer on the surface of the delaminated wafer can be surelyremoved, and the surface roughness can be improved.

[0094] There can be thus obtained the recycled (reclaimed) afer whereinthe peripheral step of the delaminated wafer and the damage layer due tohydrogen ion implantation on the delaminated surface can be removed, andthe surface roughness of the delaminated surface can be improved, sothat the surface thereof is not inferior to that of the general mirrorwafer.

[0095] Particularly, according to the present invention, thickness ofthe delaminated wafer is decreased only by 1 micron, or by 2 microns atthickest, so that the wafer itself can be reused as a bond wafer.

[0096] Namely, for example, when the SOI wafer is produced by thehydrogen ion delaminating method using the epitaxial wafer as a bondwafer, and the delaminated wafer produced as a by-product is reprocessedaccording to the present invention to reproduce an epitaxial wafer, theepitaxial wafer thus obtained can be reused as a bond wafer, so that theepitaxial layer of the expensive epitaxial wafer can be reprocessed manytimes to be reused as a SOI layer. Accordingly, a SOI wafer having highquality can be obtained at low cost.

[0097] Furthermore, when the delaminated wafer produced as a by-productin producing an SOI wafer according to a hydrogen ion delaminatingmethod in which CZ wafer is used as a bond wafer is reprocessedaccording to the present invention to produce a silicon wafer, a damagelayer on the surface of the delaminated wafer can be removed, thesurface roughness can be improved, and further, COPs in CZ wafer can beeliminated in the resultant wafer, since the wafer is subjected to theheat treatment in a reducing atmosphere containing hydrogen.Accordingly, in the case that CZ wafer is used as a bond wafer, and adelaminated wafer is CZ wafer in the hydrogen ion delaminating method,the recycle (reclaimed) wafer wherein COPs in the delaminated wafer arealso eliminated can be obtained.

[0098] Moreover, when the delaminated wafer produced as a by-product inproducing an SOI wafer according to a hydrogen ion delaminating methodin which FZ wafer is used as a bond wafer is reprocessed according tothe present invention to produce a silicon wafer, the FZ wafer having noCOP can be reprocessed many time for reuse, since the stock removal forthe delaminated wafer is kept to a minimum, so that the SOI wafer havinghigh quality can be fabricated at low cost.

[0099] The silicon wafer thus reprocessed according to the presentinvention can be used not only as a bond wafer again, but also as a basewafer, or as a general silicon mirror wafer. Namely use of the recycled(reclaimed) wafer obtained by the present invention is not limitedspecifically.

[0100] Particularly, when the epitaxial wafer is used as a bond wafer,the resultant delaminated wafer can also be used as a general epitaxialwafer.

EXAMPLE

[0101] The following examples are being submitted to further explain thepresent invention. These examples are not intended to limit the scope ofthe present invention.

Example 1

[0102] Two silicon mirror wafers having resistivity of 1.0 to 2.0 Ω·cmand a diameter of 150 mm wherein a conductive type is p type wereprepared. As for one of these wafers, an epitaxial layer having athickness of about 10 microns was grown. The epitaxial wafer was used asa bond wafer, and processed through steps (a) to (h) shown in FIG. 1according to the hydrogen ion delaminating method to fabricate SOIwafers. Thickness of SOI layer was 0.2 micron. The major processconditions used in the method were as follows.

[0103] 1) Thickness of a buried oxide layer: 400 nm (0.4 micron);

[0104] 2) Conditions of hydrogen implantation: H⁺ ions, implantationenergy of 69 kev, implantation dose of 5.5×10¹⁶/cm²;

[0105] 3) Conditions of heat treatment for delamination: in a N₂ gasatmosphere, at 500° C. for 30 minutes.

[0106] 4) Conditions of heat treatment for bonding: in a N₂ gasatmosphere, at 1100° C. for 2 hours.

[0107] The high quality SOI wafer having the SOI layer of the epitaxiallayer with thickness of 0.2 micron could be thus obtained, and thedelaminated wafer 5 was also produced as a by-product in step (e) ofFIG. 1.

[0108] The delaminated wafer was reprocessed in accordance with the step(A) to (F) of FIG. 2 in order to be reused as a bond wafer.

[0109] First, the peripheral shape of the unprocessed delaminated wafer5 shown in FIG. 2(A) was measured by being scanned with a contact probetype roughness meter. The results of the measurement was shown in FIG.3(A).

[0110] As shown in FIG. 3(A), there was formed a step 10 in theperipheral part of the delaminated wafer 5 due to the part at which thewafers were not bonded on bonding of the wafers. Height of theperipheral step 10 was more than the sum of thickness of the SOI layer(0.2 micron) and thickness of the oxide film layer (0.4 micron).

[0111] The surface roughness of the delaminated surface 11 of theunprocessed delaminated wafer 5 shown in FIG. 2(A) as measured by aphase shift interferometry method at 250 microns square and by an atomicforce microscope method at 1 micron square was 0.40 nm on the averageand 7.4 nm on average respectively (RMS value: root mean squareroughness).

[0112] The values were far worse than that of a general mirror polishedsilicon wafer. Particularly, the value at 1 micron square is 10 times ormore that of the general mirror polished silicon wafer, which shows thatthe surface roughness on the delaminated surface is locally large.

[0113] Depth of a damage layer on the delaminated surface 11 of theunprocessed delaminated wafer 5 shown in FIG. 2 (A) was determined asfollows. The wafers were subjected to etching with KOH aqueous solutionat a different etching removal. The wafers were then subjected topyrogenic oxidation at 1100° C. for one hour, and the oxide film formedon the surface was removed with an aqueous solution of hydrofluoricacid. Subsequently, the surface thereof was then subjected topreferential etching with Secco etching solution, and observed with amicroscope to measure a density of OSF (oxidation induced stackingfault) existing on the surface. Each etching removal was 0, 25, 50, 75,100, 150, 200 nm. The results of measurement was shown as a curved linea in FIG. 5.

[0114] As shown in FIG. 5, it is apparent that a damage layer having adepth of about 100 nm exists on the surface of the delaminated wafer.OSFs observed in a position deeper than 100 nm were likely generated asa result that nuclei of OSF was formed in the epitaxial layer due tocrystal defects which have been originally present in the substrateitself.

[0115] In FIG. 2(B), the delaminated wafer is dipped into hydrofluoricacid to remove the oxide film 3 on the surface. HF 50% aqueous solutionwas used as hydrofluoric acid. The peripheral shape of the delaminatedwafer wherein the oxide film was removed was measured again by scanningwith a contact probe type roughness meter. The result was shown in FIG.3(B).

[0116] As shown in FIG. 3(B), there is a step which is slightly higherthan thickness of the SOI layer (0.2 micron) in the peripheral part ofthe delaminated wafer 5.

[0117] The delaminated wafer after removal of the oxide film wassubjected to polishing to remove the peripheral step as shown in FIG.2(C). Polishing was performed under the similar condition with thesimilar apparatus to those for general polishing of a silicon wafer. Inthe present invention, the delaminated surface was polished by insertingthe delaminated wafer between polishing turn tables, rotating thepolishing turn tables in such way that each of the polishing turn tablesrotates in opposite direction each other, with loading at 500 g/cm² andwith supplying a polishing slurry to the delaminated surface to bepolished.

[0118] The result of the measurement as for the relation between thepolishing stock removal and height of the peripheral step was shown inFIG. 4. As shown in FIG. 4, it is apparent that a stock removal of about1 micron is enough to remove the steps completely. When the SOI layer isthicker and the step is higher, a stock removal of 2 micron will bepreferable for perfection.

[0119] The shape of the peripheral part of the delaminated wafer whereinthe peripheral step was removed by being polished to a stock removal of1 micron was measured again by scanning with a contact probe typeroughness meter. The result was shown in FIG. 3(C).

[0120] As shown in FIG. 3(C), the step in the peripheral part of thedelaminated wafer was completely removed by being polished to a stockremoval of 1 micron. Accordingly, the shape of the periphery of thewafer was suitable for reuse as a silicon wafer.

[0121] The delaminated wafer was subjected to cleaning before heattreatment as shown in FIG. 2(D) to prevent contamination thereof. Thecleaning is well known as RCA cleaning, and comprises two steps:(aqueous ammonia/aqueous hydrogen peroxide) and (hydrochloricacid/aqueous hydrogen peroxide).

[0122] After the cleaning before heat treatment, the delaminated waferwas subjected to the heat treatment in a reducing atmosphere containinghydrogen through use of a rapid heating/rapid cooling apparatus shown inFIG. 6 under the following condition: in hydrogen 100% atmosphere, at1200° C., for 30 seconds (FIG. 2 (E)).

[0123] The surface roughness of the delaminated surface after the heattreatment with a rapid heating/rapid cooling apparatus as measured by aphase shift interferometry method at 250 microns square and by atomicforce microscope method at 1 micron square was 0.25 nm on the averageand 0.20 nm on the average respectively (RMS value: root mean squareroughness).

[0124] The values are equivalent to those value of the general mirrorpolished silicon wafer. Accordingly, it is apparent that the surfaceroughness was significantly improved.

[0125] After the heat treatment with rapid heating/rapid coolingapparatus, OSF density on the surface as measured by subjecting thewafer to oxidation and preferential etching and being observed with amicroscope was about 10 numbers/cm², which is equivalent to the densityof OSF generated in an epitaxial layer due to influence of thesubstrate.

[0126] As described above, the delaminated wafer reprocessed accordingto the present invention has a quality good enough for reuse as anepitaxial wafer again.

[0127] Accordingly, the reprocessed delaminated wafer was used as a bondwafer as shown in FIG. 2(F). Namely, the above-mentioned recycle(reclaimed) wafer was used as a bond wafer 2 in FIG. 1(a). As thedelaminated wafer was made only 1 micron thinner by being polished toremove a peripheral step, it still has a thickness of about 9 microns.Then, the steps shown in FIG. 1 was performed to prepare SOI wafer inaccordance with hydrogen ion delaminating method. High quality SOI waferwherein SOI layer consists of the epitaxial layer could be obtainedwithout any problems.

[0128] Then, in order to determine the effect of improvement in damagelayer and the surface roughness achieved by the heat treatment in areducing atmosphere containing hydrogen, the delaminated wafer whereinthe oxide film was removed as shown in FIG. 2(B) but the polishing forremoving the peripheral step was not performed, namely the delaminatedwafer having the damage layer and the surface roughness on thedelaminated surface, was subjected to RCA cleaning described above andthen directly to the above mentioned heat treatment in a reducingatmosphere containing hydrogen through use of the rapid heating/rapidcooling apparatus shown in FIG. 6 under the following condition: inhydrogen 100% atmosphere, at 1200° C., for 30 seconds.

[0129] In order to determine the depth of a damage layer after the heattreatment, the wafers which are different from each other in etchedthickness on the surface thereof were subjected to oxidation andpreferential etching, and observed with a microscope to measure adensity of OSF existing on the surface of each wafer. Each etchingremoval was 0, 25, 50, 75, 100, 150, 200 nm. The results of measurementwas shown as a curved line b in FIG. 5.

[0130] As shown in FIG. 5, it is apparent that, even though polishingwas not performed, the damage layer on the surface of the delaminatedwafer after the heat treatment in a reducing atmosphere containinghydrogen was removed.

[0131] The surface roughness of the delaminated layer after the heattreatment with a rapid heating/rapid cooling apparatus as measured by aphase shift interferometry method at 250 microns square and by atomicforce microscope method at 1 micron square was 0.32 nm on the averageand 0.21 nm on the average respectively (RMS value: root mean squareroughness).

[0132] The values are equivalent to those of the general mirror polishedsilicon wafer. Accordingly, it is apparent that the surface roughnesswas significantly improved.

[0133] Accordingly, it is apparent that the order of the step ofpolishing for removing the peripheral step as shown in step (C) of FIG.2 and the step of heat treatment in a reducing atmosphere containinghydrogen as shown in steps (D) and (E) of FIG. 2 can be changed.

[0134] The present invention is not limited to the above-describedembodiment. The above-described embodiment is a mere example, and thosehaving the substantially same structure as that described in theappended claims and providing the similar action and effects areincluded in the scope of the present invention.

[0135] For example, the above description of the present invention hasfocused on the case where two silicon wafers are bonded to obtain an SOIwafer. However, the present invention is not limited to the embodiment.For example, the present invention is applicable to reprocessing of adelaminated wafer produced as by-product in the case where a siliconwafer after ion implantation thereinto is bonded to an insulator wafer;and a portion of the ion-implanted wafer is delaminated to therebyobtain an SOI wafer.

[0136] Furthermore, the processes of reprocessing the delaminated waferaccording to the present invention are not limited to that shown in FIG.2. Other processes such as cleaning, heat treatment or the like can beadded thereto. Furthermore, the order of the processes can be partlychanged or any of the processes can be omitted depending on the purpose.

What is claimed is:
 1. A method of recycling a delaminated waferproduced as a by-product in producing an SOI wafer according to ahydrogen ion delaminating method by reprocessing it for reuse as asilicon wafer, wherein at least polishing of the delaminated wafer forremoving of a step in the peripheral part of the delaminated wafer andheat treatment in a reducing atmosphere containing hydrogen areconducted as the reprocessing.
 2. A method of recycling a delaminatedwafer produced as a by-product in producing an SOI wafer according to ahydrogen ion delaminating method in which an epitaxial wafer is used asa bond wafer by reprocessing it for reuse as a silicon wafer, wherein atleast polishing of the delaminated wafer for removing of a step in theperipheral part of the delaminated wafer and heat treatment in areducing atmosphere containing hydrogen are conducted as thereprocessing.
 3. A method of recycling a delaminated wafer produced as aby-product in producing an SOI wafer according to a hydrogen iondelaminating method in which CZ wafer is used as a bond wafer byreprocessing it for reuse as a silicon wafer, wherein at least polishingof the delaminated wafer for removing of a step in the peripheral partof the delaminated wafer and heat treatment in a reducing atmospherecontaining hydrogen are conducted as the reprocessing.
 4. A method ofrecycling a delaminated wafer produced as a by-product in producing anSOI wafer according to a hydrogen ion delaminating method in which FZwafer is used as a bond wafer by reprocessing it for reuse as a siliconwafer, wherein at least polishing of the delaminated wafer for removingof a step in the peripheral part of the delaminated wafer and heattreatment in a reducing atmosphere containing hydrogen are conducted asthe reprocessing.
 5. The method of recycling a delaminated waferaccording to claim 1 , wherein a surface oxide film is removed beforepolishing for removal of a step in the peripheral part, as thereprocessing.
 6. The method of recycling a delaminated wafer accordingto claim 2 , wherein a surface oxide film is removed before polishingfor removal of a step in the peripheral part, as the reprocessing. 7.The method of recycling a delaminated wafer according to claim 3 ,wherein a surface oxide film is removed before polishing for removal ofa step in the peripheral part, as the reprocessing.
 8. The method ofrecycling a delaminated wafer according to claim 4 , wherein a surfaceoxide film is removed before polishing for removal of a step in theperipheral part, as the reprocessing.
 9. The method of recycling adelaminated wafer according to claim 1 , wherein a stock removal in thepolishing for removal of the step in the peripheral part as thereprocessing is 1 to 2 microns.
 10. The method of recycling adelaminated wafer according to claim 2 , wherein a stock removal in thepolishing for removal of the step in the peripheral part as thereprocessing is 1 to 2 microns.
 11. The method of recycling adelaminated wafer according to claim 3 , wherein a stock removal in thepolishing for removal of the step in the peripheral part as thereprocessing is 1 to 2 microns.
 12. The method of recycling adelaminated wafer according to claim 4 , wherein a stock removal in thepolishing for removal of the step in the peripheral part as thereprocessing is 1 to 2 microns.
 13. The method of recycling adelaminated wafer according to claim 1 , wherein the heat treatment in areducing atmosphere containing hydrogen as the reprocessing is performedat a temperature in the range from 1000° C. to a melting point ofsilicon, for 6 hours or less.
 14. The method of recycling a delaminatedwafer according to claim 2 , wherein the heat treatment in a reducingatmosphere containing hydrogen as the reprocessing is performed at atemperature in the range from 1000° C. to a melting point of silicon,for 6 hours or less.
 15. The method of recycling a delaminated waferaccording to claim 3 , wherein the heat treatment in a reducingatmosphere containing hydrogen as the reprocessing is performed at atemperature in the range from 1000° C. to a melting point of silicon,for 6 hours or less.
 16. The method of recycling a delaminated waferaccording to claim 4 , wherein the heat treatment in a reducingatmosphere containing hydrogen as the reprocessing is performed at atemperature in the range from 1000° C. to a melting point of silicon,for 6 hours or less.
 17. The method of recycling a delaminated waferaccording to claim 1 , wherein the heat treatment in a reducingatmosphere containing hydrogen as the reprocessing is performed at atemperature in the range from 1000° C. to a melting point of silicon for1 to 300 seconds through use of a rapid heating/rapid cooling apparatus.18. The method of recycling a delaminated wafer according to claim 2 ,wherein the heat treatment in a reducing atmosphere containing hydrogenas the reprocessing is performed at a temperature in the range from1000° C. to a melting point of silicon for 1 to 300 seconds through useof a rapid heating/rapid cooling apparatus.
 19. The method of recyclinga delaminated wafer according to claim 3 , wherein the heat treatment ina reducing atmosphere containing hydrogen as the reprocessing isperformed at a temperature in the range from 1000° C. to a melting pointof silicon for 1 to 300 seconds through use of a rapid heating/rapidcooling apparatus.
 20. The method of recycling a delaminated waferaccording to claim 4 , wherein the heat treatment in a reducingatmosphere containing hydrogen as the reprocessing is performed at atemperature in the range from 1000° C. to a melting point of silicon for1 to 300 seconds through use of a rapid heating/rapid cooling apparatus.21. The method of recycling a delaminated wafer according to claim 1 ,wherein said heat treatment in a reducing atmosphere containing hydrogenas the reprocessing is performed in 100% hydrogen atmosphere or a mixedatmosphere of hydrogen and argon.
 22. The method of recycling adelaminated wafer according to claim 2 , wherein said heat treatment ina reducing atmosphere containing hydrogen as the reprocessing isperformed in 100% hydrogen atmosphere or a mixed atmosphere of hydrogenand argon.
 23. The method of recycling a delaminated wafer according toclaim 3 , wherein said heat treatment in a reducing atmospherecontaining hydrogen as the reprocessing is performed in 100% hydrogenatmosphere or a mixed atmosphere of hydrogen and argon.
 24. The methodof recycling a delaminated wafer according to claim 4 , wherein saidheat treatment in a reducing atmosphere containing hydrogen as thereprocessing is performed in 100% hydrogen atmosphere or a mixedatmosphere of hydrogen and argon.
 25. The method of recycling adelaminated wafer, wherein the delaminated wafer reprocessed by themethod according to claim 1 is reused as a bond wafer of an SOI wafer.26. The method of recycling a delaminated wafer, wherein the delaminatedwafer reprocessed by the method according to claim 2 is reused as a bondwafer of an SOI wafer.
 27. The method of recycling a delaminated wafer,wherein the delaminated wafer reprocessed by the method according toclaim 3 is reused as a bond wafer of an SOI wafer.
 28. The method ofrecycling a delaminated wafer, wherein the delaminated wafer reprocessedby the method according to claim 4 is reused as a bond wafer of an SOIwafer.
 29. A silicon wafer for reuse, wherein the silicon wafer isobtained by being reprocessed according to the method of claim 1 .
 30. Asilicon wafer for reuse, wherein the silicon wafer is obtained by beingreprocessed according to the method of claim 2 .
 31. A silicon wafer forreuse, wherein the silicon wafer is obtained by being reprocessedaccording to the method of claim 3 .
 32. A silicon wafer for reuse,wherein the silicon wafer is obtained by being reprocessed according tothe method of claim 4 .